A conventional method of correcting a deviation generated between a rotational position of a synchronous motor determined from an output of a rotational position detector and an actual rotational position of the synchronous motor includes: a step of detecting a no-load state of the motor with a no-load detector; a step of detecting a motor voltage applied to a winding of the motor with a motor voltage detector; a step of computing d-axis and q-axis voltages from rotation angle information of the rotational position detector and the motor voltage; and a step of correcting the rotation angle so that the d-axis voltage becomes zero (see, for example, Patent Document 1).
In the above described conventional method of correcting the deviation generated between the rotational position of a synchronous motor determined from the output of the rotational position detector and the actual rotational position of the synchronous motor, since a motor voltage detector and a no-load detector must be provided, an apparatus adopting this method becomes expensive.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-165561